Hans Persson

Model studies of NOx storage and sulphur deactivation of NOx storage catalysts

The influence of transient changes in the gas composition on the low-temperature activity of a commercial three-way catalyst and a Pt/Al2O3 model catalyst has been studied. By introducing well-controlled periodic O2 pulses to simple gas mixtures of CO or C3H6 (in N2), a substantial improvement of the low temperature oxidation activity was observed for both catalysts. The reason for low activity at low temperatures is normally attributed to self-poisoning by CO or hydrocarbons. The improved catalytic performance observed here is suggested to origin from the transients causing a surface reactant composition that is favourable for the reaction rate.The storage of NOx under lean conditions in model NOx storage catalysts as well as the deactivation by sulphur have been studied. We find that NO2 plays an important role in the storage mechanism as an oxidising agent. Two different mechanisms for this are discussed: the formation of surface peroxides and the oxidation of nitrites to nitrates, FTIR studies show that NOx is stored as surface nitrates, The sulphur deactivation is found to be more severe when SO2 is added during the rich phase than when SO2 is added during the lean period. FTIR shows the formation of bulk sulphates both under lean and rich conditions.The mechanisms for storing of NOx in platinum-barium-alumina catalysts during lean-rich transients are investigated. Oxidation of NO to NO2 is found to be an important step. NO2 is found to be important for oxidation of the catalyst or of nitrites to form nitrates. NOx is then stored in the form of surface nitrates. FTIR studies show no formation of bulk nitrates in these experiments.

The mechanism for NOx storage

The mechanisms for storing of NOx in platinum–barium–alumina catalysts during lean–rich transients are investigated. Oxidation of NO to NO2 is found to be an important step. NO2 is found to be important for oxidation of the catalyst or of nitrites to form nitrates. NOx is then stored in the form of surface nitrates. FTIR studies show no formation of bulk nitrates in these experiments.

NOx release from a noble metal/BaO catalyst: dependence on gas composition

Automotive lean-NOx has become one of the major issues in catalysis research, due to increased environmental concerns. In the present study the NOx storage/release concept has been investigated with respect to the NOx release kinetics. The importance of the gas composition during NOx release, including the λ-value, humidity, O2, and CO2 concentration, was studied in flow reactor experiments performed with a commercial, noble metal BaO type catalyst. The main conclusions are that CO2 has a promoting and O2 a blocking effect on the NOx release.

A new approach to the electron self energy calculation

We present a new practical way to calculate the first order self energy in any model potential (local or non-local). The main idea is to introduce a new straightforward way of renormalization to avoid the usual potential expansion implying a large number of diagrams in higher order QED effects. The renormalization procedure is based on defining the divergent mass term in coordinate space and decomposing it into a divergent sum over finite partial wave contributions. The unrenormalized bound self energy is equally decomposed into a partial wave (l) sum. For each partial wave the difference is taken and the sum becomes convergent. The comparably rapid asymptotic behaviour of the method is l−3. The method is applied to lithium-like uranium, and the self energy in a Coulomb field, the finite nucleus effect and the screened self energy is calculated to an accuracy of at least one tenth of an eV.

Catalysis in the nm‐regime: manufacturing of supported model catalysts and theoretical studies of the reaction kinetics

We briefly review the methods employed to fabricate model supported nm catalysts, including wetness impregnation, vacuum vapor deposition, electron‐beam lithography, spin‐coating, and vesicle‐mediated deposition. Recent simulations of the kinetics of heterogeneous reactions occurring on supported catalyst particles are discussed as well. The attention is focused on such effects as reactant supply via the support, interplay of the reaction kinetics on different facets and edges, and adsorbate‐induced reshaping of catalyst particles.

Second-order QED corrections for few-electron heavy ions: reducible Breit-Coulomb correction and mixed self-energy-vacuum polarization correction

Some second order in alpha =e2/(h(cross)c) QED corrections are investigated for He-like heavy ions and for the 2p12/-2s shift for the Li-like ions. These corrections correspond to the reducible part of the two-photon exchange Breit-Coulomb graph and to the mixed self-energy-vacuum polarization graph. The calculations show that these corrections are important for the comparison of the recent theoretical and experimental results.

Simulations of the kinetics of rapid reactions on supported catalyst particles

Abstract Recent simulations of the kinetics of heterogeneous reactions occurring on supported catalyst particles are briefly reviewed. The attention is focused on such effects inherent for nanometer chemistry as reactant supply via the support, interplay of the reaction kinetics on different facets, and adsorbate-induced reshaping of catalyst particles. In addition, original Monte-Carlo simulations are presented illustrating the contribution of the facet edges to the reaction rate.

Monte Carlo simulations of the kinetics of catalytic reactions on nanometer-sized particles, with diffusion over facet boundaries

Using the Monte Carlo technique, we comprehensively investigate the steady-state kinetics of the 2A+B2→2AB reaction on a nanometer-sized catalyst particle. The attention is focused on the adsorbate-diffusion-mediated interplay of reaction kinetics occurring on different facets. This effect is explored for different ratios of the rate constants for adsorption, desorption, reaction, and diffusion. The results obtained demonstrate that the reaction kinetics can be uniquely different on nanometer particles compared to those on macroscopic single-crystal surfaces.

Periodic control for improved low-temperature catalytic activity

The influence of transient changes in the gas composition on the low-temperature activity of a commercial three-way catalyst and a Pt/Al2O3 model catalyst has been studied. By introducing well-controlled periodic O2 pulses to simple gas mixtures of CO or C3H6 (in N2), a substantial improvement of the low temperature oxidation activity was observed for both catalysts. The reason for low activity at low temperatures is normally attributed to self-poisoning by CO or hydrocarbons. The improved catalytic performance observed here is suggested to origin from the transients causing a surface reactant composition that is favourable for the reaction rate.

Regularization Corrections to the Partial-Wave Renormalization Procedure*

Abstract A description of the partial-wave renormalization (PWR), used for calculating the first-order self energy and certain higher-order effects for the energy levels in highly charged ions, is presented. We put special emphasis on correction terms which have to be considered due to the use of the non-covariant subtraction scheme used in PWR.